No chemical is without risk to some organism or another, even organic herbicides. There are conventional/synthetic herbicides that work well on Japanese Stiltgrass, either as a pre-emergent to prevent germination or a post-emergent to kill plants already growing. (Those active ingredients are listed in the linked page.) Organic herbicides like iron-based sprays are still being assessed for their efficacy for controlling Japanese Stiltgrass. So far, none are showing promise, at least in regards to selective action (that is, able to kill the Stiltgrass without also causing damage to the desired turf, which is what would happen when using some of the synthetic, systemic options).

The primary approach to managing summer annual weeds like Stiltgrass (since winter wipes them out for you) is to prevent seedlings from germinating in spring, and to reduce the overwintering seed bank by keeping the plants from producing seed at the end of the season (Sept-Oct), at least as much as possible. Japanese Stiltgrass can produce seed heads lower than a mower can remove them, but at least mowing will still remove most of the plant's seeding potential. All current Stiltgrass plants will die this winter, with only dormant seeds remaining in the soil. Deer and other animals can carry Stiltgrass seeds around on their fur, so if deer regularly visit the yard, consider excluding them if fencing is not cost-prohibitive.

We are not aware of any organic or lower-toxicity herbicide that works well to prevent Stiltgrass seed germination. Although corn gluten meal is an organic herbicide that can work as a pre-emergent, the application rate needed to be effective for germination prevention is higher than Maryland law permits on lawns, since the material is effectively a fertilizer in that it breaks down into a nitrogen source as it degrades. It also doesn't work reliably well, even when legal to use in non-lawn areas as a pre-emergent. Any pre-emergent used would also need to be applied about two weeks prior to when Crabgrass pre-emergent would be put down, since Stiltgrass germinates earlier than Crabgrass. (Even then, a second follow-up application several weeks later would be needed to catch any late-sprouting Stiltgrass.)

Although Stiltgrass can tolerate a sunny exposure, when it more easily overtakes turf, the turfgrass in those areas is already weakened by something, and in this case, it may be not enough sun and root competition with nearby mature trees. No turfgrass can thrive in areas that don't get enough sun (an ideal is at least 6 hours of direct summer sun, but that can vary based on grass type and other growing conditions). For those locations, it would be more practical to remove the lawn and use shade-adapted groundcovers or other plantings instead, if feasible. (This can of course be done in stages if there are cost and labor limits.) Aggressively-spreading or especially vigorous native species can more easily out-compete the stiltgrass seedlings to significantly reduce the population.

For areas of turf with about a half-day of sun, overseeding (for tall fescue, as zoysia doesn't get overseeded) now, sometime in September or early October, is a good way to improve the density of the lawn so it more easily out-competes weeds.

For the time being, manual removal in areas you are not going to use herbicide (like around the Milkweeds, for instance) is the most immediate way to remove the Stiltgrass and reduce its seed production. Soil disturbance by pulling up weeds brings more dormant weed seeds to the surface, where they may sprout (some in fall, some in spring, depending on the species), but the only other alternative for manual removal is what you're also already doing, which is mowing or cutting the plants down as low as you can, which is not good for the turfgrass.

Areas too inundated to overseed can have the sod stripped instead and be re-sodded (with tall fescue, at least, which is also more shade-tolerant, to a point, than zoysia). If it establishes well, sod will create a more instant weed-free lawn, though over time, it too will succumb to the same stressors of insufficient sunlight and tree root competition, so it's only a solution for a couple of years or so.

Miri

Miri:

If you want to put down any new grass seed this season (fall), then it would help to at least suppress Stiltgrass growth by continuing to mow. You can mow lower than usual (perhaps about 2 inches rather than 3-4 inches) to set the Stiltgrass back further while giving the newly-sown seed more light for germination, and a competitive edge on the Stiltgrass or other weeds. True, the Stiltgrass will soon be dying off, but they will continue to grow in the meantime as they attempt to produce seed.

Common Lespedeza / Japanese Clover is another summer annual lawn weed, so it too will be killed in the coming weeks as we start to freeze overnight. Since it usually grows too short to be effectively suppressed by mowing alone, and its spreading growth habit can block soil or sunlight access for newly-sown turf seed, you may want to rake/dig it out, or spot-spray with herbicide, to remove it before putting down any grass seed.

We suggest you focus overseeding efforts on this autumn (now through mid-October), not next spring. Spring seeding of cool-season turfgrasses like tall fescue is much more difficult to do successfully than in autumn. In part, this is because there will be competition pressure from spring-germinating weeds. Some pre-emergent herbicides are selective for broadleaf weeds, so they do not damage grasses. Some may be selective for warm-season grasses but not cool-season grasses, which means they can target summer annual grassy weeds and not turfgrass (excepting zoysia). Others might be risky to use withing a few weeks of seeding turf, so it will all depend on which herbicide you chose, and then follow its label directions. We don't have a database of herbicide labels to check (and manufacturers do occasionally reformulate the product or update its label), so you would need to see what's readily available, what weeds it controls, and what time constraints it may require as it pertains to when turf seed can be sown.

Miri

Can you clarify if the lawn is a blend of fescue and zoysia (you mentioned both species in the initial query) or just separate turf types in different areas of the lawn? If blended, that will complicate any herbicide use, since some chemicals affect warm-season grasses (the group to which Stiltgrass and Crabgrass belong, along with zoysia) while leaving cool-season grasses (fescue) unaffected.

Can you clarify which exact herbicide product you're considering? Brands usually have different products that may not share the same list of ingredients; the active ingredients will be listed on the label (or should be noted in the product details info. in whatever web store may be selling the product). We can't determine what impact its use may have (both efficacy for the weed control as well as potential for harm to the lawn grasses) without knowing which specific active ingredients are in the product.

It sounds like the product line you're considering is one that would be classified as a weed-and-feed, meaning it combines fertilizer with herbicide. If so, avoid using those types of products because they apply fertilizer too early in cases where an early dose of herbicide is needed to catch early-germinating weeds like Stiltgrass. Applying herbicide and fertilizer separately is the best approach so you can customize each as needed based on your unique situation.

We don't have exact soil temperature benchmarks for the germination of Japanese Stiltgrass, but Crabgrass begins to germinate once the soil (about 2 inches deep) has reached about 55 degrees F for a few consecutive days. For perspective, we're currently in the high 30s F for soil temps across central Maryland, and even with this weekend's warm-up, they are not expected to get warm enough that quickly to spur the start of Crabgrass germination. Since Japanese Stiltgrass germinates roughly two weeks prior to Crabgrass, it too is likely several weeks away from starting to sprout based on current soil temperatures.

We don't have soil temperature data regarding when Lespedeza starts to sprout, but it's likely later in spring, since Crabgrass and Stiltgrass are known as some of the earliest-sprouting summer annual lawn weeds. (Any young lawn weeds currently present or growing are likely winter annuals, which germinated last fall.)

If the manufacturer said that the product under consideration does not target either of those two weeds, Stiltgrass or Lespedeza, then the issues above might not matter as much, since it's either not going to work well, or may not work at all. The herbicide should include the target weeds (or at least very close analogs, like weeds in the same group - warm-season annual grasses, for instance) when one is chosen for application.

As we noted previously, no herbicide is without risk to pollinators, beneficial insects, and a range of other wildlife that may be present in (or forage in) the lawn. We can't generalize about the level of that risk because it depends on a wide range of factors, including herbicide ingredients, application dosage and timing, weather, and the sensitivity of the surrounding plants themselves to exposure.

Zoysia is not overseeded since it's a spreading grass, but fescue is best overseeded in autumn (every year, ideally). Was the lawn overseeded last fall? If so, that was one of the best tactics you can take for long-term competition for lawn weeds so the grass excludes most germinating seedlings on its own.

Miri

Ok, thank you for the added detail. It's surprising the zoysia has not been competitive enough to exclude enough of the Stiltgrass and Lespedeza, especially in full sun (since zoysia is known to struggle in part shade). We don't have a list of herbicide ingredients for each lawn weed, so you'd need to find a product labeled for use on both fescue and zoysia, which also targets Crabgrass (at a minimum, since herbicides that control Crabgrass generally also control Japanese Stiltgrass). A second ingredient/product might be needed for control of the Lespedeza, being a broadleaf weed. We will consult with one of our Extension weed specialists, but as she is still studying which ingredients (focused on organic options) are effective enough to use on Stiltgrass in lawns, she might not have any additional recommendations for now. We'll get back to you once we have her feedback.

Miri

We've consulted a few extension specialists, and they note that some pre-emergent herbicides (active ingredient prodiamine) control Stiltgrass reasonably well, but not all (not dithiopyr, for example). The lespedeza would need a different product as a broadleaf weed, like the post-emergent active ingredient 2,4-D.

The best long-term weed suppression (especially if you want to avoid repetitive herbicide use) approach is to replace the grass in any area that doesn't get enough sun (minimum 4-5 hours of summer sun) due to the shade from the mature trees visible in the photos. Turfgrass does not compete well with tree roots or tree shade, and even shade-tolerant grass like tall fescue and fine fescue can struggle over the long term to stay dense and vigorous enough to out-compete most weeds. If you don't need to walk on that area regularly, a grass-like alternative like various species of sedges can work well. If you need to keep the lawn for its foot traffic tolerance, then annual overseeding with tall fescue or fine fescue (though fine fescue needs to not be mown as regularly in order to thrive) in early autumn is the best way to keep it more lush.

Miri